KANSAS CITY, Mo. -- Nate Robertson is going to the bullpen to find his slider, and attempt to regain the form that made him an effective starter. Detroit manager Jim Leyland announced the move prior to Friday night's game.

Robertson gave up five homers Wednesday at Texas, tying a team record, and had only one win in his last 11 starts. He is 7-10 with a 6.09 ERA, and leads the American League in earned runs allowed (101) while being tied for the most homers allowed with 25.

"Skip's got a job to do and has to make the best decision for the team," Robertson said, "and he can't feel good about putting me out there with what happened in Texas. I don't feel good about going out with what happened in Texas."

The five homers by the Rangers came in a stretch of 13 at-bats when a double and triple also were hit. Leyland was asked if logic dictated the removal of Robertson from the rotation.

"It's not a good thing to do right now," he said. "He was going out there without his slider. That's his big weapon and he doesn't have it. So, as we speak, he is in the bullpen."

Leyland said he was not sure who will start in Robertson's place Tuesday against Cleveland in Detroit. Asked if Dontrelle Willis, 3-1 with a 4.28 ERA in five starts with Toledo, is a possibility, Leyland said, "As we speak today, the answer is 100 percent no. It's not the right thing to do and Dave agrees with it."

General manager Dave Dombrowski and Leyland discussed the pitching decisions, and Leyland noted that Willis was "methodical and not very good" in allowing five walks in five innings in his Wednesday start.

And so the Tigers have two left-handed starters making a combined $50.25 million for this year and the next two who are not in the rotation. Robertson signed a three-year, $21.25 million extension before this season, and was asked if he placed pressure on himself as a result.

"The team obviously thought highly enough to give me a three-year deal like that," Robertson said, "and I want to give back. To not consistently produce bothers me. There is pressure regardless."

Leyland said Robertson would work on his mechanics with pitching coach Chuck Hernandez, noting that his first relief appearances "would not be in do-or-die situations."

Robertson last pitched in relief in two games in 2004, but his other 160 appearances for the Tigers have been starts. He is third among league pitchers with 152 starts since 2004, and fifth in the league with 929 innings in that time.

"I've always been proud of not missing starts," Robertson, 30, said.

Now his stuff isn't reliable enough to keep him in the rotation.

"It's tough, the lowest point for me," he said. "I think I've kind of bottomed out."

Robertson said the flat slider has left him susceptible to left-handed hitters for the first time in his career. Lefties batted .247 against him prior to this year, but are hitting .315 against him in 2008. He had allowed only 10 homers in 720 at-bats by lefties before this season, when they have hit eight in 197 at-bats.

Leyland said Robertson had some tendinitis in his knees and "hip issues" but added, "by (Robertson's) own admission he doesn't feel that restricts him."

Robertson, though, said he needs to work on his flexibility in the offseason, and said he would get on a program to improve that with team strength coach Javair Gillett.

But for now his focus is the return of the sharp-breaking slider.

"First you have to be comfortable in your mechanics and then you have to get results," Robertson said. "There is no timetable. If I can finish out this year contributing on a positive note -- that is what I want."